Corrugated paperboard has gained wide-spread acceptance in the packaging industry because of its strength, its cost advantages, and its ability to protect the contents of the package. One of the best known types of corrugated packaging is a corrugated box.
A corrugated box is formed from a sheet of corrugated material that is cut, folded, and stapled or glued (where necessary) to create the desired finished shape of the box. The manufacture of a corrugated box often occurs in several steps performed by different people. A first company will often manufacture the corrugated sheets. The corrugated sheets are then placed in stacks and shipped to a printer, who prints the flat sheets. The printed corrugated sheets can then be tendered to a converter who folds, cuts, and staples the sheet to convert the flat sheet into a three dimensional package, such as a corrugated box.
As shown in FIG. 1, a single wall corrugated sheet 10 includes a top liner 12, a bottom liner 14, and a medium 16 that is also referred to as a "flute". The flute 16 extends between the top liner 12 and the bottom liner 14. The liners 12, 14, and flute 16, are usually made from a heavy paper known in the industry as "kraft" paper. High speed corrugating machinery (not shown) heats, moisturizes and glues three layers of kraft paper together so that the top and bottom liners 12, 14 are planar and the medium ("flute") layer 16 is curved in a sinusoidal pattern. High speed, automated equipment is also used to cut the sheets to their desired size, and then stack the sheets onto a pallet for shipment.
The corrugated paperboard industry manufactures corrugated sheets 10 in a wide variety of "thickness" sizes. However, the industry has also adopted several "standard thicknesses" used by packagers to identify the particular characteristics of the sheet that they desire. To some extent, the differences in thicknesses result from the thicknesses of the liners 12, 14 that are used, although, to a larger extent, thickness differences result from the height, and manner in which the medium layer 16 is fluted. The thickness "A" of a particular corrugated sheet 10 is referred to as its "caliper". By convention, the caliper A is measured to include the thickness of the entire corrugated sheet 10, including the top liner 12 and bottom liner 14.
Currently, several standards exist that have gained wide-spread acceptance in the industry. These various standards are known as "A-flute", "B-flute", "C-flute", "E-flute", and "F flute". Of these, A flute is generally the thickest, and F flute (also known as micro-flute) is the thinnest.
Due to the mechanical tolerances of the corrugating machines themselves, variations in caliper occur in each flute type. Generally, these variations are tolerated by the industry, as they are usually only on the order of a few thousandths of an inch. Variations in sheet caliper also occur due to the uneven moisturizing and heating of the kraft paper, the uneven amounts of glue used to bond the separate layers together, and the uneven basis weight of the kraft paper.
In FIG. 2, a double-wall corrugated sheet 20 is shown that consists of five layers of kraft paper, including a generally planar top liner 22, a generally planar middle liner 24, and a generally planar bottom liner 26. A first fluted layer 28 extends between the top liner 22 and the middle liner 24, and a second fluted layer 30 extends between the middle liner 24 and the bottom liner 26. The overall caliper of the double wall corrugated sheet is comprised of the addition of the caliper B of the upper layer of the double walled corrugated sheet, and the caliper C of the bottom layer of the double walled corrugated sheet.
The choice of particular calipers to be joined together (e.g. A flute and C flute) depends upon the desires of the manufacturer and purchaser. Generally any combination of flute size (e.g. A flute and C flute; B flute and C flute; C flute and E flute, etc.) can be used to create double walled corrugated sheets. Double walled corrugated sheets 20 are similar to single corrugated sheets 10, insofar as variations exist within the standard caliper for each type of flute combination. For example, an A/B flute caliper from one batch may have a greater or lesser thickness (caliper) than an A/B flute sheet from another batch, due to differences in paper, glue, etc.
Just as the individual sheets, e.g. 10, 20, are not consistent from batch to batch, the stacks in which the sheets are placed often contain substantial inconsistencies, and do not represent perfect arrays of corrugated sheets. Turning now to FIG. 3, a stack 40 of corrugated sheets is shown being placed on top of a pallet 42. The stack 40 includes sheets 44, 46, 48, 50, 52, 54, and 58. Sheets 44, 46, 50, 54 and 58 are all positioned similarly, so that their edges are aligned at the "nominal edge" of the stack of 40. However, the edge of sheet 48 is recessed inwardly from the nominal edge, and the edge of sheet 52 protrudes outwardly from the edge. Additionally, a gap 60 exists between sheets 58 and 54 that is devoid of material. Such gaps 60 are quite common, and are caused by the fact that the sheets are often not perfectly planar. This non planarness is referred to as "warpage."
After the stack of sheets is discharged from the stacking machinery, human operators visually inspect the stack for certain errors in the desired quality of the sheets, and manually remove from the stack, any sheets that do not meet quality standards. Also, an operator may choose to restack one or more stacks manually on the roller/conveyor. Thus, it is unlikely that several stacks, which have been consecutively discharged by the stacking machinery, will consist of exactly the same number of individual sheets. As such, there could be a substantial variation in the number of sheets within any particular stack.
To achieve proper inventory control at the production facility and appropriate customer invoicing, it is desirable to determine the exact count of individual sheets in each particular stack of sheets which are produced by the sheet manufacturer.
Several methods of counting sheets are known. In particular, three general methods exist. The first method involves the use of a human operator who measures the height of the stack of corrugated sheets with a tape measure, and then derives the number of sheets contained in the stack by dividing the height of the stack by the thickness of a particular sheet. A second method for counting sheets involves linear displacement, and a third involves the use of optical devices having a limited focal range.
The first method involving the human operator who measures the height of a stack with a tape measure has some inherent problems that may induce error. This method is prone to human error such as misreading the tape measure, calculator key punching errors and errors made in copying the total sheet count onto a paper shipping ticket. Additionally, variations in actual sheet thickness also induces errors. Although the variations in the thickness of particular sheets from batch to batch is usually small, and within acceptable tolerances, the aggregate variation that would exist in a stack containing a large number of sheets may be sufficiently great so as to cause a stack of particular size (e.g. 72 inches) to contain substantially more or less sheets than another stack of the same size, even if the individual sheet variance between the two stacks thickness is small. Another error in the human method is caused by the non-planarness (warpage) of the sheets in a stack. This warpage can cause gaps between adjacent sheets. If a sufficient number of gaps exist in a stack, then the sheet count can be miscalculated significantly.
The second method relates to the use of linear displacement to count the number of corrugated sheets in a stack. A stack counting apparatus using such a linear displacement technique is disclosed in Williamson et al U.S. Pat. No. 4,417,351. Williamson discloses the use of a movable platen that senses the total height of a stack by applying pressure to the top of the stack to compress the gaps that are created by the non-planarness (warpage) of the sheets. With a separate sensor, the device determines the thickness of a single sheet, and then calculates the total number of sheets by dividing the height of the stack by the thickness of a single sheet. While the induced compression makes this method less error prone than the first method, minor variations in the thickness of the corrugated board produced at the corrugating machinery can, when multiplied by many sheets in the stack, result in an erroneous total calculated sheet count. Additionally, this method requires a human operator to insert a single sheet into the thickness sensor manually for each stack to be counted, resulting in time delays and extra labor costs for the production facility.
The third known prior art method involves the use of an optical device having a limited focal range, such as the device disclosed in Woodward U.S. Pat. No. 5,040,196. Woodward discloses an optical device that is held in physical contact with the side of a stack of sheets, and then moved perpendicularly, by a human operator, from the bottom of the stack to the top. Woodward's device must be calibrated for the particular board thickness (flute) before using it to count the sheets in the stack. Therefore, it is possible that an operator must recalibrate the device between successive counting operations on stacks of different flute types. This calibration can be time consuming and costly. Another difficulty is that the Woodward device must be held in physical contact with the side of the stack. As such, it is believed that the Woodward device may have difficulty counting sheets within a stack that are not at the nominal edge of the stack, such as protruding sheet 52 (FIG. 3) and recessed sheet 48. Protruding sheets would be especially problematic, as they would tend to act as a barrier against the vertical movement of the device.
Another drawback with the Woodward device is that since it is hand-held, it requires a human operator to position the device in physical contact with the stack and move the device at a uniform speed from the bottom to the top of the stack.
Additional prior art devices for counting sheets are shown in Gersl U.S. Pat. No. 4,331,879 and Adabisch U.S. Pat. No. 4,296,314.
Although the foregoing devices may perform their intended functions in a workman-like manner, room for improvement exists. In particular, room for improvement exists in providing a more highly automated device that is capable of accurately counting corrugated sheets, in a stack of sheets that are stacked in a less than perfect manner. It is therefore one object of the present invention to provide such a device.